Isolated Conus Medullaris Infarction: Rare Cases Highlighting Diagnostic Challenges and Clinical Insights

Yuri Je; Seokho Choi; Han-Jin Cho; Young-Eun Park

doi:10.18214/jend.2023.00185

Abstract

Spinal cord infarction that exclusively affects the conus medullaris is exceptionally rare. The dominant symptoms include flaccid paraparesis, sensory deficits with or without saddle anesthesia, and neurogenic bladder, all of which mimic cauda equina syndrome. We report two cases where patients initially presented with sudden onset of leg monoparesis, sensory deficits, and voiding difficulties. Although initially suspected of having lumbosacral radiculopathy or cauda equina syndrome, they were later diagnosed with conus medullaris infarction, as evidenced by their clinical course and spine magnetic resonance imaging findings. This report provides detailed clinical information about conus medullaris infarction, supplemented by a review of the literature, to aid in the diagnosis of this condition.

Keywords: Spinal cord; Infarction; Paraparesis; Urinary bladder, neurogenic; Cauda equina

Introduction

Spinal cord infarction is a rare condition, accounting for less than 1% of all stroke causes [1]. It typically results from vascular occlusion of the anterior spinal artery, which occurs more frequently than occlusion of the posterior spinal artery [2]. The anterior spinal arteries originate from the anterior segmental medullary arteries, with the artery of Adamkiewicz being the largest and most significant contributor to spinal cord infarction. This artery primarily supplies the lower two-thirds of the spinal cord, making infarctions more common in the lower thoracic and lumbar regions [2]. The onset of spinal cord infarction is rapid, typically reaching its most severe point within minutes to 12 hours. Symptoms often include motor weakness and a dissociative sensory deficit, characterized by a loss of pain and temperature sensation. While hyperreflexic deep tendon reflexes and pathological reflexes are frequently observed, there are instances where these are absent, and the presentation may resemble spinal shock [1,2]. In spontaneous cases of spinal cord infarction, most patients exhibit vascular risk factors such as hypertension and diabetes. The condition is also commonly associated with aortic dissection or aortic surgery [1].

The conus medullaris is the terminal segment of the spinal cord before it transitions into the cauda equina. Its blood supply is primarily derived from the artery of Adamkiewicz, yet isolated infarctions of the conus medullaris remain exceedingly rare. Typical symptoms of conus medullaris infarction include paraparesis, sensory deficits, and difficulties with voiding and defecation, which are also common in cauda equina syndrome [3]. These overlapping symptoms can lead to misdiagnosis, prompting unnecessary diagnostic and therapeutic interventions. It is crucial for physicians to understand the clinical presentation, etiology, and diagnostic approaches for conus medullaris infarction to avoid inappropriate treatment, although the scarcity of cases limits the availability of data.

This report describes two cases of conus medullaris infarction, detailing their clinical manifestations, courses, and outcomes, and reviews the literature on conus medullaris infarction.

Case Reports

1) Case 1

A 68-year-old man presented with acute-onset weakness in the right lower leg on the day of admission. He also experienced difficulty urinating. His symptoms remained stable and did not worsen after their onset. Two years prior, he had suffered a left basal ganglia infarction, from which he partially recovered, allowing him to walk with the aid of a cane. Since then, he had been prescribed clopidogrel and aspirin. During the initial neurological examination, reduced strength was observed solely in the right distal lower limb; ankle dorsiflexion and plantar flexion were rated as 4 on the Medical Research Council (MRC) scale, and great toe flexion and extension were rated as 1. Decreased sensation to touch and temperature was noted on the outer side of the right lower leg and the dorsum of the right foot. Both urinary dysfunction and decreased anal tone were confirmed. Deep tendon reflexes were brisk only in the right knee, and no pathological reflexes were observed.

Lumbosacral radiculopathy was initially suspected due to neurological deficits localized to one side of the lower limb, accompanied by ipsilateral sensory disturbances indicative of a lesion in the right L5 root. Additionally, a combined cauda equina lesion was considered due to the patient's voiding difficulties. Consequently, lumbar magnetic resonance imaging (MRI) was performed. The MRI revealed high signal intensities in the conus medullaris on T2-weighted images, although there were no abnormalities in the cauda equina (Fig. 1A, B). Laboratory findings were generally unremarkable, except for an elevated erythrocyte sedimentation rate (35 mm/h; normal range 0 to 10). Both nerve conduction studies and needle electromyography showed normal results, with no abnormalities detected in somatosensory evoked potential testing.

Based on the acute onset and stable course of symptoms, along with MRI findings, the patient was diagnosed with conus medullaris infarction. He continued on his existing regimen of aspirin and clopidogrel while under observation. Subsequently, there was a rapid improvement in the muscle strength of the right ankle and toes, occurring within approximately 10 days of hospitalization, which enabled stable ambulation without assistance. However, bladder dysfunction persisted, as evidenced by sensory loss in the bladder during urodynamic testing.

2) Case 2

A 66-year-old man presented with sudden onset of weakness in his left lower limb one day prior to admission. There was no visible movement below the left ankle, and he experienced difficulty with urination. His symptoms did not worsen after their onset. Several hours before the symptoms appeared, he reported a dull, aching pain in his buttocks. His medical history included diabetes and hyperlipidemia. During the initial neurological examination, ankle dorsiflexion, plantar flexion, and extension and flexion of the great toe were all graded as 0. There was bilateral loss of touch and temperature sensation below the L5 level, along with loss of anal sphincter tone and bladder sensation. Deep tendon reflexes were normal, and no pathological reflexes were detected.

The initial suspicion of lumbosacral nerve root involvement was due to the motor weakness being limited to the unilateral distal leg. Additionally, the presence of urination difficulty led to the consideration of a combined cauda equina lesion. MRI of the lumbar spine showed high signal intensities in the conus medullaris, predominantly on the left side, with no contrast enhancement observed (Fig. 1C, D). Laboratory results were generally normal, except for an elevated blood glucose level (153 mg/dL; normal range 70 to 110). While nerve conduction studies and electromyography showed normal results, somatosensory-evoked potential testing indicated delayed cortical responses on both sides.

Based on the acute onset and progression of his symptoms, he was diagnosed with conus medullaris infarction, and treatment with clopidogrel was initiated. On the third day of hospitalization, there was a sudden decrease in the strength of his right ankle dorsiflexion and plantar flexion to grade 4, and great toe flexion and extension to grade 3. In response, aspirin was added to his treatment regimen, and no further deterioration has been reported since then. His deep tendon reflexes remained normoactive thereafter. Three months after discharge, he was able to walk with the aid of a cane, but only partial recovery of urinary dysfunction was observed.

This study was approved by the Institutional Review Board (IRB) of Pusan National University Hospital (IRB No. 2307-020-129) and granted an exemption from written consent by the IRB.

Discussion

The two cases are characterized by the sudden onset of unilateral weakness in the distal lower extremity, accompanied by sensory deficits and urinary dysfunction. These symptoms suggest involvement of the lumbosacral nerve roots or cauda equina. In the first case, sensory symptoms in the dermatome of the ipsilateral lower lumbar nerves led to a suspicion of lumbosacral radiculopathy or cauda equina syndrome. However, MRI of the lumbar spine ruled out cauda equina lesions in both cases. Importantly, the rapid progression to the nadir of symptoms at onset is a key indicator of vascular diseases. In this context, high signal intensities in the conus medullaris are indicative of conus medullaris infarction, and the neurological deficits observed in the two patients correlate with lesions in the conus medullaris. However, the presence of sensory symptoms on the same side as the weakness in the first patient casts some doubt on the diagnosis of conus medullaris lesions. This could suggest involvement of the right corticospinal tract and the left spinothalamic tract. Another possibility is that, in addition to the conus medullaris lesion, there may have been a concurrent nerve root infarction due to occlusion of radicular arteries, as reported in previous cases [4,5]. This was not confirmed in our case, but it is plausible given that radicular arteries, along with segmental medullary arteries, originate from segmental spinal arteries [4].

Isolated conus medullaris infarction is exceedingly rare within the category of spinal cord infarctions. This rarity is attributed to the rich vascular anastomotic network between the anterior and posterior spinal arteries in the conus medullaris, which provides a protective environment against ischemic damage [6]. Due to this rarity, the literature on conus medullaris infarction is limited, making it a challenging diagnosis. According to previous reports, the most common presentation is bilateral flaccid paralysis with sensory deficits and a neurogenic bladder [3]. Although asymmetrical weakness was not uncommon (7/19, 36.8%), monoparesis as an initial presentation was reported to be quite rare (1/19, 5.3%) [7]. Furthermore, the monoparesis observed in our cases became a significant factor in initially overlooking conus medullaris infarction. Pain preceding neurological symptoms, as seen in the second case, was commonly reported in the buttocks, low back, and lower limbs [7,8]. The risk factors for conus medullaris infarction are often not apparent. However, the presence of preceding stroke, diabetes, and hyperlipidemia are considered underlying risk factors and are mentioned as influencing prognosis, with studies reporting a correlation with poor walking ability outcomes [3,7,8]. Our cases showed a relatively favorable prognosis in terms of walking recovery. However, it is notable that in both cases, improvement in bladder function was noted to be slow.

In general, spinal cord infarction on MRI may exhibit owl-eye or pencil-like hyperintensities on T2-weighted images, with approximately 30% of cases appearing normal in the early stages, according to various reports [9]. Diffusion-weighted imaging (DWI) can reveal significant diffusion coefficient restriction in about two-thirds of cases, making it highly valuable for confirming the diagnosis [8,9]. Although DWI was not performed in this instance, as infarction was not initially suspected, the rapid diagnosis provided by this imaging technique is crucial for initiating appropriate treatment early, potentially preventing further symptom deterioration and improving prognosis. The authors present these cases to enhance the understanding of conus medullaris infarction as a rare cause of acute lower limb paralysis. Through a review of the literature and an analysis of clinical symptoms, progression, and outcomes, we aim to provide insights that may aid in future diagnoses.